1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a plurality of developing units which are used selectively.
2. Description of Related Art:
There has been known an electrophotographic image forming apparatus equipped with a plurality of developing units for producing color copies in addition to monochromic copies, which are used selectively by operating selection keys provided on a control panel to operate desired developing units. In such an image forming apparatus, operation of a print key to command the image forming apparatus to start the copying operation is invalid while the developing unit changeover operation is executed in response to the operation of a developing unit selecting key. Therefore, the print key must be operated to start the copying operation after the developing unit changeover operation has been completed. Furthermore, once an incorrect developing unit selection signal is entered by operating the selection key to select a wrong developing unit, the developing unit selecting procedure needs to be repeated again to select a correct developing unit after the completion of the developing unit changeover operation for selecting the wrong developing unit, because the developing unit changeover operation is started upon the entry of a developing unit selection signal by operating the selection key. Several seconds necessary for the developing unit changeover operation is a significant loss in operating time which cannot be ignored, and the unnecessary repetition of the developing unit changeover operation entails the useless wear of the components.
Still further, in the conventional image forming apparatus of such a type, the changeover of the developing unit, for example, from a developing unit for the monochromatic copying operation by use of black developer to a developing unit for the color copying operation by use of color developer requires a so-called magnetic brush removing operation for recovering black developer remaining on the developing sleeve before starting the color copying operation. Imperfect removal the residual developer causes muddy color copies with black developer and hence it is impossible to produce clear color prints. Accordingly, a fixed time on the order of several seconds is allocated for the developing unit changeover operation so that the developer remaining on the developing sleve is removed completely.
Accordingly, when the change over of the developing unit is required in such a conventional image forming apparatus, a time necessary for the developing unit changeover operation is spent unavoidably before operating the print start key to start the printing operation, which has been an impediment to efficient copying operation.
Furthermore, in the conventional image forming apparatus, the developing unit changeover operation can be interrupted by operating an interrupt key when the operator becomes aware that the developing unit changeover operation is unnecessary after the developing unit changeover operation has been started. However, when the developing unit which has been operative before the interrupted developing unit changeover operation is selected again, the entire developing unit changeover operation requiring a predetermined time is repeated. In such a case, the developer remaining on the developing sleeve is recovered to some extent during the interrupted developing unit changeover operation and hence the subsequent developing unit changeover operation can be accomplished in a time shorter than the predetermined time for the normal developing unit changeover operation. Nevertheless, the developing unit changeover operation subsequent to the interruption of the preceding developing unit changeover operation is continued for the predetermined time for the normal developing unit changeover operation before the operation of the print start key becomes valid unnecessarily wasting time.